1. Field
The disclosed subject matter relates to devices and methods for coating of conductive plastic and other types of parts. More particularly, the disclosed subject matter relates to connecting and grounding mechanisms and associated methods for use with conductive plastic parts that are to be electromotively coated.
2. Brief Description of the Related Art
The term “electromotive” coating process as used herein refers to any coating process in which an electrical potential exists between the part being coated and the coating material. Examples of electromotive coating processes include electrostatic coating, electrodeposition (“e-coat”) processes, electromotive vapor deposition, and electroplating processes. The part may be painted or coated with any suitable water-based or organic-based composition (or water/organic mixture) including conductive primer compositions which further enhance the electronic conductivity of the article, or with a solventless organic composition by a powder coating or vapor deposition method.
In electrostatic coating, the coating or fluid (e.g., paint) is typically atomized, then negatively charged. The part to be coated is electrically neutral, making the part positive with respect to the negative coating droplets. Thus, the coating particles are attracted to the surface of the part and held there by the charge differential until cured.
When an electrostatic spray gun is used, droplets of fluid/paint pick up a charge from an electrically charged electrode located on the gun (usually at the tip of the spray nozzle of the gun). The charged droplets are discharged from the gun by a combination of fluid pressure and air pressure.
Electrostatic spraying offers high transfer efficiency and excellent edge coverage. The attraction between charged paint or fluid droplets and the part can be strong enough to cause paint overspray to curve back and adhere to the part, which results in more efficient and consistent coating.
Electrodeposition (e-coat) techniques are also known in the art. In general, organic coatings can be applied to conductive substrates such as metals or conductive plastics by electrodeposition. An aqueous media containing the organic coating to be applied is formed and is commonly referred to as an e-coat bath. The aqueous media typically contains from about 5% to about 30% solids.
An organic polymer is dispersed throughout the e-coat bath and may be positively or negatively charged. Coating of the part is accomplished by immersing the part in the aqueous media and applying a current to the part. The charged polymers are attracted to the material (part) to be coated, which has the opposite charge. The organic particles are then deposited onto the part where they coalesce forming a coating. The part is removed from the e-coat bath rinsed, and the coating is cured. Curing the organic coating to form a final film aids in making the coating tough, resistant to water and organic solvents, etc.
Electrophoretic deposition systems are characterized as either anodic or cathodic thereby indicating the function of the part to be coated. Where the part to be coated is the cathode, the system is referred to as a cathodic system. Similarly, where the part to be coated is the anode, the system is referred to as an anodic system.
Drawbacks with electromotive systems relate to the ability to maintain the part, and especially conductive plastic parts, at a particular electrical potential. In addition, certain problems can arise when intermittent or unwanted electrical or mechanical connections between any of the components of the coating system occur. For example, unwanted discharge of electrical potential can occur and variations in the electrical field during the coating process can occur due to the unwanted contact, or inadequate connection, which can lead to both quality and safety problems.
A drawback unique to electrostatic applications is that the process may not efficiently coat recessed areas due to what is known as Faraday cages (the charged droplets tend to be attracted to the sides of the recess and sharp edges instead of penetrating to the bottom). In addition, electrically conductive materials that are located near the spray area, such as the supply mechanisms, containers, and spray equipment should be grounded to prevent static buildup, to prevent unwanted electrical discharge, and to provide a quality coating. All mechanisms that carry or hold the parts should be kept clean to ensure conductivity to ground. Charges that build up on ungrounded surfaces may discharge or cause inefficient or deficient coating results. Thus there has been a long felt need in the art to provide consistent and positive connection between the part that is being coated and the carrying or holding mechanisms.
In some typical coating systems, parts to be coated are transported through a coating zone or bath by a mechanical conveyor. The parts can be constructed of a conductive material such as metal or conductive plastics. The conveyor can be made from similar materials and should be maintained at or near the potential of the part to be coated which, at least for some electrostatic processes, can be at the electrical ground potential. The parts to be coated are supported on the conveyor by hangars which maintain the parts close together and at the potential of the conveyor. The coating dispensing device can be a gun, as described above for electrostatic processes, or can be another type of spray nozzle, and includes a mechanism for electrically charging the dispensed particles of coating material to a relatively high potential, typically a high negative potential, with respect to the potential of the conveyor and hangars. Alternatively, the coating dispensing device can be a bath into which the hangars direct the part to be coated.
Intermittent contact or high-impedance contact between the parts to be coated and the conveyor can cause an electrical potential change and result in unevenly coated parts and a waste of coating material. This can occur because the charged coating material, which should migrate along electric field lines from the high potential dispensing device to the different potential maintained on the parts, instead, migrates along field lines established between the dispensing device and other articles and surfaces in the vicinity of the dispensing device which are maintained at low-magnitude potential, such as the conveyor itself. Thus, many of the benefits associated with electromotive coating are lost if the parts to be coated are not properly grounded or synchronized at a particular potential to terminate the electrostatic field.
The need for easily cleaned and operationally efficient hangars that suspend parts to be coated from conveyors that convey the parts past or through coating dispensing equipment is well established. Several types of such equipment are known, for example, the devices illustrated and described in U.S. Pat. No. 4,450,954 and U.S. Patent Publication No. 2004/0154536, the disclosures of which are hereby incorporated by reference in their entirety.
There has been a long felt need in the art to provide a ground mechanism that is sufficiently robust to efficiently maintain ground or other different potential for a part during electromotive coating. In addition, there remains a need to provide a more stable and predictable connection between a hangar or other connection device and a part to be coated. The connection should be such that relative electrical potential between all apparatus, including connecting devices, parts, paint or fluid spray, coating equipment, bath, etc., could be maintained and reproduced with ease during manufacture.